Fat Marmot Population Explodes

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Fat Marmot Population Explodes

In a remote valley in the mountains of Colorado, the marmot population has tripled over the past decade, but this may not ultimately be good news for the fat, furry, squirrel-like creatures.

Increasingly, short winters have meant that yellow-bellied marmots (Marmota flaviventris) near the Rocky Mountain Biological Laboratory now emerge 20 days earlier from their seven- to eight-month hibernation than they did in the late '70s. This, in turn, has meant more time to get fat over the summer, less fat loss over the winter and, over the past decade, a huge spike in their survival and reproductive success.

"We believe that gradual change in climate crosses a threshold, and causes abrupt changes in population," said biologist Arpat Ozgul from the Imperial College of London, lead author of a study on the marmots being published July 21 in Nature.

While these marmots have been getting gradually fatter since 1976, as far back as the study's analysis went, the population didn't explode until 2000. Between 2000 and 2008, their numbers increased by 18 percent per year, which is very rapid for a mammal.

Researchers found a strong correlation between body mass and individual survival and reproduction, but are still working out why there was such a sudden change.

What sets this study apart is that the researchers have been tracking individual marmots (by painting different symbols on their backs, such as lightning bolts or diamonds) over the entire course of their lifetimes, for more than 30 years.

"This study presumably started before the impacts of climate change was a major motivation," said biologist Murray Humphries from McGill University. "Now, 33 years later we have really important, detailed information on how climate change ripples into population impacts."

While getting fatter and having a population boom sounds like a marmot heyday, warmer weather might not be so great for the cold-adapted alpine creatures in the long run.

"We strongly suspect that this is only a short-term response," said Ozgul. "“Marmots are adapted to cold environments and have a small tolerance for heat. Long-term response depends whether the summer droughts will increase. Warmer summers will limit their food resources, which could make them more susceptible to predators and less likely to gain enough weight before the next hibernation.”

"In the past couple years we've already started to see higher rates of predation, taking into account that there are more marmots overall," Ozgul added.

"I'd be surprised if the population keeps growing, even if the summers keep getting longer," echoed Humphries. "What goes up must come down."